# Selecting antenna for rocket telemetry system

I'm currently in a rocket team and we are building a rocket that can reach 3000m (10.000 feet). I am responsible to find an antenna for the telemetry system. I was searching on the net to find the best antenna but I didn't understand most of the technical terms. Can someone answer these questions? Thanks :)

1. We are using a 433 MHz RF module. Is the antenna should be 433 MHz or can it be more? Can I use 868 MHz antenna?
2. Is 433 MHz RF Module enough for the 10.000 feet? (The distance could be more than 10K feet.)
3. There are some details of our RF Module. I found this antenna but I'm not sure that it can work at 10.000 feet (The total distance could be more than 10K feet. We are assuming that distance is 20K feet). Can someone say to me his/her thoughts?

RF MODULE DETAILS

https://www.m2mmarket.com.tr/e32-ttl-1w-302

ANTENNA DETAILS

https://www.m2mmarket.com.tr/U151,36,2j010-868-mhz-rf-anten-2j.htm

• Hello, and welcome to this site! Do any of these related questions have an answer that will work for you? – Mike Waters Feb 14 at 18:22
• "3000km": jaw dropped! Then I read 10k feet. – Juancho Feb 14 at 22:18
• It's wrong to consider an antenna designed for 868 MHz as "more" than one designed for 433 MHz. – mike65535 Feb 15 at 14:35
• If you are satisfied with one of the answers, please indicate by clicking the "check" mark. This helps housekeeping by removing your question from the "unanswered" list. – Brian K1LI Feb 17 at 11:30

You should use an antenna that is designed for the frequency of your RF module unless you have a compelling reason to choose a different antenna. A properly designed antenna will make the best use of the RF module's output power and avoid damage to the RF module.

If you decide to use an antenna designed for a different frequency range from the RF module, you should probably design and insert an impedance-matching network between the RF module and the antenna to protect the RF module from power reflected by the mismatched antenna. Here's an example from the datasheet for a PCB-mounted chip antenna:

The chip antenna is the 4.5-mm x 13-mm rectangle on the right-hand side of the diagram. The black rectangles in the "Matching Circuit" are surface-mount (SMT) inductors and/or capacitors in a $$\pi$$-network configuration. The rectangle at the bottom of the "Matching Circuit" is a ground pad with vias that pass through the PCB to make a low-impedance connection to the copper plane on the back side.

Well-known electronic parts distributors sell a variety of monopole, helical monopole, dipole, patch and chip antennas that might work in your application. Your antenna of choice must survive high acceleration in atmosphere and add as little drag as possible to the rocket assembly. The size, shape and mounting arrangement will all be important. An antenna with very low gain (large negative dB value) and/or low efficiency will not radiate the signal from the RF module as well as an antenna with more gain and higher efficiency.

The application notes section of the Linx Technologies web page contains numerous useful documents with which you could educate yourself. I refer you, in particular to AN-00501: Understanding Antenna Specifications and Operation, which discusses all of the important factors you need to consider when choosing your antenna.

I provide the links only as examples of your antenna options. I am not affiliated with any distributor or manufacturer.

• An antenna on a rocket has no "ground" reference. You might avoid monopole type antennas that rely on a ground plane...the requirement of a ground plane is often not mentioned. Dipole antenna might be preferable - it is complete on its own. – glen_geek Feb 15 at 17:27
• @glen_geek Good point. The cited app note describes how to create the required reference conductor to make the monopole antenna work properly. – Brian K1LI Feb 15 at 17:39
• We're only talking 4 miles line of sight, so high gain doesn't really need to be a priority. I was involved in a launch where the payload reached 180,000 feet and used a dipole at 440MHz. – Duston Feb 19 at 15:22

It may be possible to use a conductive surface mounted on the outside of the rocket body as an antenna. Not knowing the dimensions of your rocket and without the benefit of any design equations, I used EZNEC+ to create a crude model of a cavity-backed cylindrical slot antenna:

The "cylinder" is about 6.5-m tall and 920-mm in "diameter," the "slot" is about 4.2-m tall and 381-mm wide, producing this SWR curve with a 200-ohm reference impedance:

"Shortening" the slot while maintaining its "width" raises the resonant frequency and the feedpoint impedance. A "shorter" and "narrower" slot will produce a match at the desired frequency.

The radiation is strongest and fairly omnidirectional off the ends of the cylinder:

• What would the polarization be? I imagine it would be difficult to match it if the rocket starts to roll. Is there a way to adapt this idea to make the signal circularly polarized? Maybe wrap a spiral of wire around the outside of the rocket body? – mrog Feb 19 at 22:36
• @mrog I've seen several cavity-backed slot antennas with crossed slots to achieve circular polarization. Not sure how to implement that on this form factor. – Brian K1LI Feb 20 at 1:07